macOS - improve demangle to emit create events

[bpasero]

Fixes #31

The current implementation of demangle has a flaw: some operations from the macOS finder can result in the birthtime getting restored as it was (e.g. undo a delete or copy a folder). As such, nsfw often would wrongly only send a change event when it is a create event.

For some operations we can tell that the event should be create, so a boolean flag is added. The current behaviour of sending a change event is still preserved, so we might be ending up sending one event too much, but I think that is totally fine.

This slightly improves the situation for #146 in that a create event is fired for the renamed file, though we miss the delete event. I think detecting a case rename will probably be tricky for demangle because the stat check will not figure out that the old name was removed from disk.

[bpasero]

Nevermind, turns out I did not fully understand how events work on macOS, specifically it looks like macOS decides to report repeated created events even when the file is modified.

To properly detect whether a file is modified or created, a different strategy is needed, such as tracking all stats of reported paths, similar to: https://github.com/parcel-bundler/watcher/blob/728b44aeec7801bae9b11e02a7fb3f0bdf94b924/src/macos/FSEventsBackend.cc#L91